A very popular and useful wrench is a socket wrench of the type which has a handle, a head at one end of the handle with a socket mounting stub, and a set of interchangeable socket members of different sizes which are adapted for individual selective mounting on the stub. Usually called "socket wrench sets", they are commonly packed in a box which has a compartment for the handle and a compartment, or separate compartments, for the interchangeable socket members. In addition, it is quite common for such a socket wrench set to include an extension member which has one end portion that seats on the stub and a second end portion comprising a stub to selectively receive the interchangeable socket members.
Socket wrench sets are often used in connection with service work on machinery or vehicles; and it is a common experience of a person doing such work to find that he needs to change socket members when the case containing them is not readily at hand; and this causes undesirable delay and, sometimes, frayed nerves.
Many interchangeable socket members are lost because they are not immediately returned to the case; especially when, for example, the person removes from the case two or three socket members that he expects to need on a particular job and then misplaces one or more of them.